In structural engineering, there is often the need to measure the tension in a bolt or another fastener or a set of fasteners (now done by measuring the torque applied to tighten a bolt) and also a need to measure the tensile force applied to structural members under tension. An example of torque measurement is a bulkhead secured by a pattern of bolts; an example of tensile force measurement is a cable under tension in the truss span of a suspension bridge.
Existing force measuring devices measure applied forces by sensing the flexure of parts of their own structure. Such devices are complex, expensive, delicate, and require frequent calibration. For example, U.S. Pat. No. 4,899,600 discloses a force transducer which includes a reference member fixed at one end and free at the other end forming a cantilever beam. This transducer measures force by the flexure of this cantilever beam. U.S. Pat. No. 4,572,006 discloses two flat spring elements and a differential capacitor detection system for measuring the magnitude of forces applied to the flat springs. This device senses the flexure of the flat springs. To other patents, U.S. Pat. Nos. 4,448,085 and 4,558,600, also use flexure to measure the forces inducing it. The primary reason flexure based devices are used is because a greater deflection results from flexure from a given force than from pure tension or compression; thus more sensitivity results in the readout. Although flexure results in greater sensitivity, strength and durability in the measuring device is sacrificed.
Although these transducers operate on the principal of detecting the change in a longitudinal gap between a flexible member and a reference member, they do not lend themselves to the measurement of a full range of applied compressive or tensile forces and they cannot be applied to fasteners or structural members under compression or tension in the field, i.e., in situ.
Moreover, many of these devices which operate on the principal of differential capacitance involve a fairly intricate arrangement of cantilever beam elements and capacitance sensor circuitry. The accuracy of some of these devices is highly affected by temperature changes and most of these devices do not work reliably in adverse environments.
Other than torque wrenches which can be inaccurate and strain gauges which are susceptible to the effects of the environment and require repeated calibration, there are no suitable force measurement devices which measure the compressive and tensile forces applied to structural fasteners and members in situ.
In engineered structures, bolted joints are the weakest part of most assemblies. Maximum efficiency and fatigue life can be achieved by torquing the fasteners to a value slightly less than their tensile yield point. The amount of torque applied to achieve this level of tension is currently measured by torque wrenches whose accuracy is dependent on the lubricity between the mating threads of the fasteners. This dependency introduces inaccuracy which can result in insufficiently tightened bolts or incipient bolt failure and thus a weakened structure.